The present invention provides benzylated amine Mannich base compositions, epoxy curing agents and amine-epoxy compositions derived from such compounds, and articles produced from such compounds and/or compositions.
Epoxies are known for their excellent adhesion, chemical and heat resistance, good-to-excellent mechanical properties, and good electrical insulating properties. Cured epoxy resin systems have found extensive applications ranging from coatings, adhesives, and composites. Specific examples include epoxy composites using carbon fiber and fiberglass reinforcements, protective coatings for metal surfaces, and construction products for concrete, cementitious or ceramic substrates, often referred to as civil engineering applications, such as formulations for concrete flooring.
Cured epoxy resin systems consist of two components that chemically react with each other to form a cured epoxy, which is a hard, duroplastic material. The first component is an epoxy resin and the second component is a curing agent, often referred to as a hardener. Epoxy resins are substances or mixtures which contain epoxide groups. The curing agents include compounds which are reactive to the epoxide groups of the epoxy resins, such as amines, carboxylic acid, and mercaptans (H. Lee and K. Neville “Handbook of Epoxy Resins” McGraw Hill, New York, 1967, pages 5-1 to 5-24). The epoxy resins may be cross-linked or cured by curing agents. The curing process is the chemical reaction of the epoxide groups in the epoxy resins and the reactive groups in the curing agents. The curing converts the epoxy resins, which have a relatively low molecular weight, into relatively high molecular weight materials by chemical addition of the curing agents to the epoxy resins. Additionally, the curing agent may contribute to many of the properties of the cured epoxy.
With the constant need in the coatings industry to comply with stricter volatile organic content (VOC) regulations, many two component epoxy systems now require the use of low molecular weight epoxy resins which are liquids at room temperature and free from solvents. The use of these resins can lead to several application and performance problems, for example, in coating applications, amine-epoxy compositions based on liquid resins tend to cure much more slowly than a comparable solvent based solid epoxy resin formulation and this problem becomes more severe at lower cure temperatures. Shipyards, for example, often reside in locations with cold winters, and paint must be applied when temperatures are at or below 5° C. Certain amine-epoxy formulations cure very slowly at these temperatures, often requiring at least 24 hours cure before reaching a point where coated structures can be handled or recoated with a second or third coating. In addition, at these application temperatures, many epoxy coatings suffer from problems referred to in the industry as blush, carbamate and exudate during the curing process.
These problems, in part are due to the incompatibility of the amine curing agent and epoxy resin, which cause phase separation and result in amine migration to the coating surface. In primary amines, the migratory amine can react with carbon dioxide (CO2) and moisture present in the air resulting in carbamation. Whether in the form of carbamation or the greasy surface layer, referred to as exudate, these surface defects detract from the appearance of the coating and can lead to intercoat adhesion failures if the initial coating is overcoated. These problems are generally worse for coatings applied and cured under conditions of extreme cold and high humidity, where the amine-epoxy compatibility within the coating matrix is reduced.
Even with good compatibility between amine and epoxy, many epoxy curing agents suffer from slow cure at low temperature and only incomplete cure takes place. Slow cure requires longer time for coating to set and dry which means longer time to return to service or for the subsequent layer to be coated.
There are numerous amine-based curing agents and amine-epoxy compositions that are employed in the amine-epoxy coating industry; including polyamides, amine adducts and Mannich bases (including phenalkamines), however, none of these known products completely addresses the needs or solves the problems noted above. U.S. Pat. No. 4,269,742 discloses the preparation and use of Mannich base compounds as epoxy hardener to produce tack-free films at low temperature. U.S. Pat. No. 6,465,601 discloses Mannich base compounds as accelerators for curable epoxy and polyurethane systems. U.S. Pat. No. 8,735,512 discloses the preparation and use of Mannich base compounds based on N,N′-dimethyl secondary diamine polymers.
A Mannich base is a reaction product of an aldehyde, a phenol compound, and an amine. While certain Mannich bases can be used in amine-epoxy formulations, they are also not without drawbacks. Mannich bases can provide the productivity benefits of faster cure, however, the nature of the amines employed in such compositions means certain Mannich bases still suffer from blush, carbamation and exudate formation. In addition, depending upon the synthesis route adopted, the resultant Mannich base compound may contain unacceptable amounts of residual phenol. Phenol is a toxic chemical and its presence at levels greater than 1% in a chemical mixture can require special handling and disposal procedures to protect workers and the environment.
It has now been found that the benzylated Mannich base compositions of the present invention described herein below are suitable for use as curing agents in curable epoxy systems to improve or overcome the incompatibility between amine and epoxy, and the slow amine epoxy reaction rate at low temperature. They are readily miscible with the epoxy resins and positively influence the properties of the cured systems. The benzylated Mannich base compositions, according to the present invention, may also have the advantage of comparatively low viscosity. The lower viscosity reduces the need to use organic compounds to adjust the viscosity, thus reduces the use of volatile organic compounds (VOC) to comply with governmental VOC regulations. In addition, the new benzylated Mannich base compositions are also free from residual phenol and thus provide added environmental and worker safety benefits.
The disclosure of the foregoing publications, including patents and patent applications, is hereby incorporated by reference.